This invention relates to phase shift photomask blanks in photolithography with short wavelength (i.e., &lt;400 nm) light. More specifically this invention relates to phase shift photomask blanks that attenuate and change the phase of transmitted light by 180.degree. relative to light propagating the same path length in air. Such photomask blanks are commonly known in the art as attenuating (embedded) phase shift photomask blanks or half-tone phase shift photomask blanks. Still more particularly, this invention proposes novel attenuating embedded phase-shift photomask blanks, whose optical properties can be engineered at any wavelength by multilayering ultrathin UV transparent layers, periodically or aperiodically, with ultrathin UV absorbing layers.
The electronics industry seeks to extend optical lithography for manufacture of high density integrated circuits to critical dimensions of 0.25 mm and smaller. To achieve this, lithographic photomask blanks will need to work with short wavelength light, i.e. &lt;400 nm. Two wavelengths targeted for future optical lithography are 248 nm (KrF laser wavelength) and 193 nm (ArF laser wavelength). A phase shift mask enhances the patterned contrast of small circuit features by destructive optical interference.
The concept of a phase shift photomask blank that attenuates light and changes its phase was revealed by H. I. Smith in U.S. Pat. No. 4,890,309 ("Lithography Mask with a p-Phase Shifting Attenuator"). Known attenuating embedded phase shift photomask blanks fall mainly into two categories: (1) Cr-based photomask blanks containing Cr, Cr-oxide, Cr-carbide, Cr-nitride, Cr-fluoride or combinations thereof; and (2) SiO.sub.2 - or Si.sub.3 N.sub.4 -based photomask blanks containing SiO.sub.2 or Si.sub.3 N.sub.4 together with a predominantly opaque material, such as MoN or MoSi.sub.2. Commonly the latter materials are referred to generically as `MoOSiN`. Cr-based photomask blanks have the advantage that they are chemically durable and can use most of the familiar processing steps developed for opaque Cr photomask blanks. The second category of photomask blanks based on SiO.sub.2 - or Si.sub.3 N.sub.4 exploit their transparency into the deep UV and ease of dry etching with more innocuous fluorine-base chemistry. However, the need to develop photomask blanks for even shorter wavelengths (&lt;200 nm) renders Cr chemistries less desireable because photomask blanks based exclusively on Cr (i.e., oxides, nitrides, carbides, fluorides or combinations thereof) are too optically absorbing at such wavelengths. The disadvantage of `MoSiON` photomask blanks in this short wavelength regime is that they are too Si-rich and consequently have poor etch selectivity relative to the quartz (SiO.sub.2) substrate. Thus, they require an etch stop, an additional layer of a material which etches poorly in a fluorine etchant.
In addition, there are references in the literature to attenuating embedded phase shift photomask blanks comprising hydrogenated amorphous carbon layers, tantalum and its compounds with a layer of Cr metal, or one or more layers composed of a hafnium compound.
Practical phase shift photomask blanks require tailorable transmissivity at the operating wavelength (&lt;400 nm) and at the inspection wavelength (typically 488 nm). Additional desirable properties include electrical conductivity for permitting electron-beam patterning, dry-etchability with selectivity to the photoresist and the quartz substrate, environmental, chemical, and radiation stability. It would also be advantageous, if the same process could be used to fabricate photomask blanks with the needed optical properties at different wavelengths.
It is common in the art to modify the chemistry of the topmost layer of, or add a layer to a binary or phase shift photomask blank, so that it is anti-reflective or more chemically robust. Although these photomask blanks then contain "multiple" layers in the sense that they have at least two layers, these additional layers do not tailor the optical transmissivity and transmitted phase of the photomask blank.